Damped biphasic energy delivery circuit for a defibrillator

ABSTRACT

A defibrillator capable of delivering a damped biphasic truncated (DBT) defibrillation pulse is provided. An energy storage circuit is coupled across a high voltage switch such as an H-bridge for delivering a defibrillation pulse to the patient through a pair of electrodes. A controller operates to control the entire defibrillation process and detects shockable rhythms from the patient via an ECG front end. The energy storage circuit consists of an energy storage capacitor, a series inductor, a shunt diode, and optionally a resistor in series with the inductor. The controller measures as the patient dependent parameter the time interval between the initial delivery of the defibrillation pulse and the occurrence of the peak current or voltage to determine the first and second phases of the defibrillation pulse to provide for compensation for patient impedance. Other types of patient dependent parameters, measured either before or during delivery of the DBT defibrillation pulse, could be alternatively employed to achieve the impedance compensation.

BACKGROUND OF THE INVENTION

[0001] This invention relates to electrotherapy circuits and inparticular to a defibrillator which is capable of applying dampedbiphasic defibrillation pulses to a patient.

[0002] Electro-chemical activity within a human heart normally causesthe heart muscle fibers to contract and relax in a synchronized mannerthat results in the effective pumping of blood from the ventricles tothe body's vital organs. Sudden cardiac death is often caused byventricular fibrillation (VF) in which abnormal electrical activitywithin the heart causes the individual muscle fibers to contract in anunsynchronized and chaotic way. The only effective treatment for VF iselectrical defibrillation in which an electrical shock is applied to theheart to allow the heart's electro-chemical system to re-synchronizeitself. Once organized electrical activity is restored, synchronizedmuscle contractions usually follow, leading to the restoration ofcardiac rhythm.

[0003]FIG. 1 is an illustration of a defibrillator 10 being applied by auser 12 to resuscitate a patient 14 suffering from cardiac arrest. Incardiac arrest, otherwise known as sudden cardiac arrest, the patient isstricken with a life threatening interruption to their normal heartrhythm, typically in the form of ventricular fibrillation (VF) orventricular tachycardia (VT) that is not accompanied by a palpable pulse(shockable VT). In VF, the normal rhythmic ventricular contractions arereplaced by rapid, irregular twitching that results in ineffective andseverely reduced pumping by the heart. If normal rhythm is not restoredwithin time frame commonly understood to be approximately 8 to 10minutes, the patient 14 will die. Conversely, the quicker defibrillationcan be applied after the onset of VF, the better the chances that thepatient 14 will survive the event. The defibrillator 10 may be in theform of an automatic external defibrillator (AED) capable of being usedby a first responder. The defibrillator 10 may also be in the form of amanual defibrillator for use by paramedics or other highly trainedmedical personnel.

[0004] A pair of electrodes 16 are applied across the chest of thepatient 14 by the user 12 in order to acquire an ECG signal from thepatient's heart. The defibrillator 10 then analyzes the ECG signal todetect ventricular fibrillation (VF). If VF is detected, thedefibrillator 10 signals the user 12 that a shock is advised. Afterdetecting VF or other shockable rhythm, the user 12 then presses a shockbutton on the defibrillator 10 to deliver the deliver the defibrillationpulse to resuscitate the patient 14.

[0005] The patient 14 has a transthoracic impedance (“patientimpedance”) that spans a range commonly understood to be 20 to 200 ohms.It is desirable that the defibrillator 10 provide animpedance-compensated defibrillation pulse that delivers a desiredamount of energy to any patient across the range of patient impedancesand with a peak current limited to safe levels substantially less than amaximum value.

[0006] The minimum amount of patient current and energy delivered thatis required for effective defibrillation depends upon the particularshape of the defibrillation waveform, including its amplitude, duration,shape (such as sine, damped sine, square, exponential decay). Theminimum amount of energy further depends on whether the current waveformhas a single polarity (monophasic), both negative and positivepolarities (biphasic) or multiple negative and positive polarities(multiphasic).

[0007] If the peak current of the defibrillation pulse that is deliveredto the patient 14 exceeds the maximum value, damage to tissue anddecreased efficacy of the defibrillation pulse will likely result. Peakcurrent is the highest level of current that occurs during delivery ofthe defibrillation pulse. Limiting peak currents to less than themaximum value in the defibrillation pulse is desirable for both efficacyand patient safety.

[0008] Most external defibrillators use a single energy storagecapacitor charged to a fixed voltage level resulting in a broad range ofpossible discharge times and tilt values of the defibrillation pulseacross the range of patient impedances. A method of shaping the waveformof the defibrillation pulse in terms of duration and tilt is discussedin U.S. Pat. No. 5,607,454, “Electrotherapy Method and Apparatus”,issued Mar. 4, 1997 to Gliner et al. Using a single capacitor to providethe defibrillation pulse at adequate energy levels across the entirerange of patient impedances can result in higher than necessary peakcurrents being delivered to patients with low patient impedances. At thesame time, the charge voltage of the energy storage capacitor must beadequate to deliver a defibrillation pulse with the desired amount ofenergy to patients with high patient impedances.

[0009] Various prior art solutions to the problem of high peak currentsexist using resistance placed in series with the patient 14 tocompensate for variations in patient impedance. In U.S. Pat. No.5,514,160, “Implantable Defibrillator For Producing A Rectangular-ShapedDefibrillation Waveform”, issued May 7, 1996, to Kroll et al., animplantable defibrillator having a rectilinear-shaped first phase uses aMOSFET operating as a variable resistor in series with the energystorage capacitor to limit the peak current. In U.S. Pat. No. 5,733,310,“Electrotherapy Circuit and Method For Producing Therapeutic DischargeWaveform Immediately Following Sensing Pulse”, issued Mar. 31, 1998, toLopin et al., an electrotherapy circuit senses patient impedance andselects among a set of series resistors in series with the energystorage capacitor to create a sawtooth approximation to a rectilinearshape in the defibrillation pulse. Using current limiting resistors tolimit peak current as taught by the prior art results in substantialamounts of power being dissipated in the resistors which increases theenergy requirements on the defibrillator battery. Furthermore, suchprior art circuits require complex, active control systems to regulatethe current during the delivery of the defibrillation pulse.

[0010] The use of inductors in the energy storage circuit along with theenergy storage capacitor to shape the defibrillation pulse is well knownin the art. The basic RLC defibrillator topology is explained in U.S.Pat. No. 4,168,711, “Reversal Protection for RLC Defibrillator”, issuedSep. 25, 1979 to Cannon, III et al. RLC defibrillators utilize aninductor in series with the energy storage capacitor to deliver a dampedsine wave defibrillation pulse. Such waveforms are typically nottruncated and the discharge time is on the order of 50-60 milliseconds(ms). RLC defibrillator designs according to the prior art do notaddress the problem of limiting peak currents or otherwise compensatingfor the range of patient impedances.

[0011] More recent biphasic defibrillator designs such as theHeartstream Forerunner® automatic external defibrillator (AED) utilizesolid state switches such as silicon controlled rectifiers (SCRs) andinsulated gate bipolar transistors (IGBTs) connected in an H-bridge toproduce a biphasic truncated exponential (BTE) defibrillation pulse.Such solid state switches require snubber circuits in series with theenergy storage capacitor to control the rate of change of voltage orcurrent through the switches to prevent switch damage as well as toprevent false triggering from transient energy. The snubber circuit inthe Forerunner AED employs a 150 microhenry (uH) inductor. Similarly, inU.S. Pat. No. 5,824,017, “H-Bridge Circuit For Generating A High-EnergyBiphasic Waveform In An External Defibrillator”, issued Oct. 20, 1998,to Sullivan et al., a protective element having resistive and inductiveproperties is interposed between the energy storage capacitor and the Hbridge. Sullivan et al teach that the protective element 27 is used tothe limit the rate of change of voltage across, and current flow to, theSCR switches of the H bridge. However, snubber circuits, while designedto protect the switch components of the H-bridge, do not address theproblem of limiting peak current to the patient across the range ofpatient impedances.

[0012] It would therefore be desirable to provide a defibrillator thatdelivers an impedance-compensated defibrillation pulse to the patientwith limited peak currents.

SUMMARY OF THE INVENTION

[0013] A defibrillator capable of delivering a current-limiteddefibrillation pulse is provided. An energy storage circuit charged to ahigh voltage by a high voltage charger circuit which receives its energyfrom a battery. The energy storage circuit is coupled across a highvoltage switch such as an H-bridge for delivering a defibrillation pulseto the patient through a pair of electrodes. A controller operates tocontrol the entire defibrillation process and detects shockable rhythmsfrom the patient via an ECG front end.

[0014] The energy storage circuit consists of an energy storagecapacitor, a series inductor, a shunt diode, and optionally a resistorin series with the inductor. The series inductor has an inductance valuechosen to limit the peak current of the defibrillation pulse deliveredto the patient for the lowest expected value of patient impedance. Theinductance value is chosen as a function of the capacitance value andcharge voltage of the energy storage capacitor. The series resistor maybe added depending on the internal resistance of the series inductor.Alternatively, the series inductor may be modeled as an ideal inductorand the series resistor represents the effective series resistance (ESR)of the series inductor. The shunt diode is necessary to clamp thevoltage generated by the energy stored in the series inductor becausethe defibrillation pulse is truncated by the high voltage switch. Thewaveform developed according to the present invention is a dampedbiphasic truncated (DBT) waveform which is distinct from the biphasictruncated exponential (BTE) waveform of the prior art.

[0015] The controller uses the current and voltage information suppliedby the energy storage circuit to determine a patient dependentparameter. In the preferred embodiment, the patient dependent parameteris measured during the delivery of the defibrillation pulse.Alternatively, the patient dependent parameter may be measured beforedelivery of the defibrillation such by the use of a low level signal orthe delivery of a non-therapeutic pulse to the patient.

[0016] A patient dependent parameter is a measurement of time, voltageor current that is directly related to the patient impedance for thegiven combination of capacitance, inductance, and series resistance.This patient dependent parameter can be used by the controller to setthe time of the first and second phases of the defibrillation pulse tocontrol the amount of energy delivered to the patient. One such patientdependent parameter that can be determined in the case of the DBTdefibrillation pulse is the measured time interval between the initialdelivery of the defibrillation pulse to the time that the current or thevoltage peaks. Other patient dependent parameters, such as thepercentage voltage drop across the patient, percentage voltage dropacross the energy storage capacitor, or measured time to reach a circuitcharge delivery using a current integrator, may also be effectivelyused.

[0017] One feature of the present invention is to provide adefibrillator that delivers current limited defibrillation pulses.

[0018] A further feature of the present invention is to provide adefibrillator that delivers damped biphasic truncated (DBT)defibrillation pulses.

[0019] Another feature of the present invention is to provide an energystorage circuit capable of delivering current limited defibrillationpulses.

[0020] A further feature of the present invention is to provide a methodof delivering damped biphasic truncated defibrillation pulses.

[0021] Other features, attainments, and advantages will become apparentto those skilled in the art upon a reading of the following descriptionwhen taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is an illustration of a defibrillator being applied by auser to resuscitate a patient suffering from cardiac arrest;

[0023]FIG. 2 is a simplified block diagram of a defibrillator accordingto the present invention;

[0024]FIG. 3 is a schematic diagram of the energy storage circuit of thedefibrillator of FIG. 2;

[0025]FIG. 4A-C are graphs comparing the typical waveshapes of abiphasic truncated exponential (BTE) defibrillation pulse generatedaccording to the prior art with the damped biphasic truncated (DBT)defibrillation pulse generated according to the present invention;

[0026]FIG. 5 is a graph of the DBT defibrillation pulse generatedaccording the present invention; and

[0027]FIG. 6 is a flow diagram of the method of generating a DBTdefibrillation pulse according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0028]FIG. 2 is a simplified block diagram of a defibrillator 10according to the present invention. The pair of electrodes 16 forcoupling to the patient 14 are connected to an ECG front end 18 andfurther connected to an HV switch 28. The ECG front end 18 provides fordetection, filtering, and digitizing of the ECG signal from the patient14. The ECG signal is in turn provided to a controller 26 which runs ashock advisory algorithm that is capable of detecting ventricularfibrillation (VF) or other shockable rhythm that is susceptible totreatment by electrotherapy.

[0029] A shock button 30, typically part of a user interface of thedefibrillator 10 allows the user to initiate the delivery of adefibrillation pulse through the pair of electrodes 16 after thecontroller 26 has detected VF or other shockable rhythm. A battery 24provides power for the defibrillator 10 in general and in particular fora high voltage charger 22 which charges the capacitors in an energystorage circuit 20. Typical battery voltages are 12 volts or less, whilethe energy storage circuit 20 may be charged to 1500 volts or more. Acharge voltage control signal from the controller 26 determines thecharge voltage in the energy storage circuit 20.

[0030] The energy storage circuit 20 is connected to the RV switch 28which operates to deliver the defibrillation pulse across the pair ofelectrodes 16 to the patient 14 in the desired polarity and durationresponse to the switch control signal from the controller 26. The HVswitch 28 is constructed using an H bridge to deliver biphasicdefibrillation pulses in the preferred embodiment but could readily beadapted to deliver monophasic or multiphasic defibrillation pulses andstill realize the benefits of the present invention.

[0031] In FIG. 3, there is shown a simplified schematic of the energystorage circuit 20. An energy storage capacitor 50 stores the energy forthe defibrillation pulse and is typically charged to over 2,000 volts inthe preferred embodiment. The energy storage capacitor 50 is connectedto the HV switch 28 through a current sense resistor 52 on one lead andthrough a series inductor 54 and series resistor 56 on the other lead. Ashunt diode 58 is coupled in shunt across the series inductor 54 andseries resistor 56. The HV switch 28 connects the energy storage circuit20 to the pair of electrodes 16 in a selected polarity and for aselected duration responsive to the switch control signal from thecontroller 26. The shunt diode 58 is necessary to include in the energystorage circuit 20 because the defibrillation pulse is truncated,leaving a sudden interruption in the current path. The shunt diode 58becomes forward biased upon truncation in order to shunt the voltagegenerated by the energy stored in the series inductor 54.

[0032] A voltage measurement circuit 62 is connected across the energystorage capacitor 50 to measure the charge voltage and provide a voltagesignal back to the controller 26. The voltage measurement circuit 62could be implemented using a voltage divider network to scale thevoltage down to lower level signal that is then supplied to an analog todigital converter (ADC) which provides the voltage signal to thecontroller 26 in the form of digital measurement values.

[0033] A current measurement circuit 60 is connected across the currentsense resistor 52 to measure the current flowing from the energy storagecapacitor 50. The current sense resistor 52 has a relatively smallresistance value that does not interfere with operation of the energystorage circuit 20. The current measurement circuit 60 may include anADC to provide the current signal to the controller 26 in the form ofdigital measurement values. The current measurement circuit 60 may alsoinclude an integrator circuit to determine the amount of charge that hasbeen discharged from the energy storage capacitor 50 if required formeasuring a patient dependent parameter. Such an integrator could be ananalog integrator constructed using well known techniques usingoperational amplifiers and capacitors. Depending on the method ofmeasuring a patient dependent parameter used for controlling the firstand second phase durations of the defibrillation pulse, either thevoltage measurement circuit 62 or the current measurement circuit 60could be omitted from the energy storage circuit 20. The use of the DBTdefibrillation pulse according to the present invention allows for aselected energy level, such as 150 joules, to be delivered with anacceptable level of accuracy, to a patient of unknown impedance withinthe range of 20 to 200 ohms, and with both pulse duration and peakcurrent within predetermined limits.

[0034] The choice of capacitance value and charge voltage for the energystorage capacitor 50, inductance value for the series inductor 54, andresistance value for the series resistor 56 to achieve the DBTdefibrillation pulse with the desired characteristics requires areasonable amount of experimentation. The DBT defibrillation pulse musthave the desired limit on peak current for low patient impedances. Atthe same time, the DBT defibrillation pulse should provide an adequatetime duration between the initialization of the defibrillation pulse andthe peak current or peak voltage to allow for accurate measurement ofthe patient dependent parameter if that patient dependent parameter ischosen for the control method.

[0035] The following tables provide the results of various combinationsof component values and charge voltages that could be chosen to achievea DBT defibrillation pulse. TABLE 1 Combinations of Component Values Rseries L series Capacitance Charge Stored Combination (ohms) (mH) (uF)Voltage (V) Energy (J) 1 10 10 100  1800 160 2 10 20 100  1800 160 3 1020 70 2150 160 4 20 20 70 2300 185

[0036] TABLE 2 Peak current and energy delivered for each combinationfrom Table 1. Patient Combination Combination Combination CombinationImpedance 1 2 3 4 25 ohms Peak 44 41 46 41 Current (A) Energy 108 108109 96.7 Delivered (J) 50 ohms Peak 28 27 31 29 Current (A) Energy 127127 127 124.3 Delivered (J) 100 ohms Peak 16 15 18 18 Current (A) Energy138 138 138 145 Delivered (J) 175 ohms Peak 9.8 9.6 11.5 11.5 Current(A) Energy 144 144 144 156.2 Delivered (J)

[0037] All delivered energy calculations assume that approximately 95%of the energy stored in the energy storage capacitor 50 is delivered tothe patient 14. Taking combination 4 as an example, omitting the seriesinductor 54 to obtain a BTE defibrillation pulse would result insubstantially higher peak currents of 51 A for the 25 ohm patientimpedance and 33 A for the 50 ohm patient impedance compared with 41 Aand 29 A respectively according to the DBT defibrillation pulse of thepresent invention.

[0038] The data from Table 2 show that inductance values in the 10 to 20mH range for the combinations 1-4 deliver acceptable results in limitingpeak current for low patient impedances while providing adequate energydelivery for high patient impedances. The inductance value of the seriesinductor 54 may be in the range of 5 to 100 mH in conjunction with theenergy storage capacitor 50 with a capacitance of 50 to 100 uF toachieve the DBT defibrillation pulse according to the present invention.Inductance values than 5 mH tend to provide insufficient limiting of thepeak current to the patient 14. Inductance values greater than 100 mHrequire inductors that are physically too large for portabledefibrillator applications.

[0039]FIG. 4A-C are graphs (not to scale) of patient current versus timefrom initial delivery of the defibrillation pulse comparing the typicalwaveshapes of a biphasic truncated exponential (BTE) defibrillationpulse generated according to the prior art with the damped biphasictruncated (DBT) defibrillation pulse generated according to the presentinvention.

[0040] In FIG. 4A, there is shown a graph of a typical BTE waveformdrawn as trace 70. In the first phase spanning time T0 to T2, the peakcurrent occurs at or very close to time T0, with an exponential decay inthe patient current that is truncated at time T2. Truncation means thatthe defibrillation pulse is switched off while a substantial amount ofcurrent is continuing to flow to the patient 14. The second phase spansfrom time T3 to T5 with a peak current at or very close to time T3followed by exponential decay to time T5 where the second phase istruncated.

[0041] In FIG. 4B, there is shown a graph of a typical DBT waveformaccording to the present invention drawn as trace 72. The truncationtimes T2 and T5 are aligned for reasons of comparison to show how theBTE and DBT waveforms are roughly equivalent in the amount of energydelivered. In the first phase, the peak current now occurs at time T1which is substantially later than time T0 and at a value substantiallyless than peak current of the BTE waveform of FIG. 4A. The decay is nolonger exponential and substantially flatter than the BTE waveform.Similarly in the second phase, the peak current occurs at time T4 whichis substantially later than time T3 and at a value substantially lessthan the peak current at time T3 of the BTE waveform.

[0042] In FIG. 4C, there is shown a graph of the traces 70 and 72superimposed on each other to visually compare the BTE and DBTwaveshapes. It has been discovered that the area 74 between the traces70 and 72 which represents the energy delivered in the peak current hasrelatively little therapeutic value to the patient 14. This energy istherefore wasted and a potentially harmful to the patient 14 if the peakcurrent of the BTE waveform exceeds the maximum value. Limiting the peakcurrent according to the present invention may have the additionaladvantage of allowing the use of higher charge voltages on the energystorage capacitor 50 while decreasing its capacitance value createsadvantages in terms of reduced physical size and weight.

[0043] In FIG. 5, there is shown a graph of a typical DBT waveshapeillustrating a method of controlling the durations of the first andsecond phases. It is well known that the amount of energy delivered by adefibrillation pulse varies by patient impedance and can be controlledby appropriate setting of the duration time of the first and secondphases. A patient dependent parameter which correlates with patientimpedance can be measured, either before or during the delivery of thedefibrillation pulse, and then used to determine the first and secondphase durations.

[0044] As shown in FIG. 5, a method of controlling the first and secondphase durations of the DBT defibrillation can be based on a measurementof the time interval between time T0, the initial delivery of thedefibrillation pulse, and time T1, the time that the peak currentoccurs. This value of this time interval directly depends on the patientimpedance which can then be used as a patient dependent parameter todetermine the first phase duration spanning time T0 to T2 and the secondphase duration spanning time T3 to T5. It will be noted that this methodwill work equally well by measuring the voltage across the pair ofelectrodes 16 and measuring the time interval to the peak voltage.

[0045] In this way, the patient dependent parameter is determined duringthe delivery of the defibrillation pulse. This method has the advantageof eliminating the separate step of determining the patient dependentparameter prior to delivery of the defibrillation pulse. Measuring thepatient dependent parameter prior to delivery of the defibrillationpulse may be also be used using existing circuitry which detects patientcontact across the pair of electrodes 16 and still gain the advantagesof the DBT defibrillation pulse according to the present invention.

[0046] In FIG. 6 there is shown a flow diagram of a process ofdelivering an impedance-compensated defibrillation pulse by thedefibrillator 10 based on one embodiment of the present invention thataddresses the time interval to peak current. Another embodiment which issubstantially similar is the time interval to peak voltage.

[0047] In step 200 labeled BEGIN DELIVERY OF FIRST PHASE OFDEFIBRILLATION PULSE TO PATIENT, the delivery of the defibrillationpulse is initiated by the controller 26 which controls the HV switch 28using the switch control signal. In the typical scenario such as in anAED, the pair electrodes 16 have been placed on the patient 14, thecontroller 26 has detected a shockable rhythm such as VF, and the user12 has pressed the shock button 30 to deliver the defibrillation pulse.The controller 26 then begins the process of delivering thedefibrillation pulse and functions as a control system to provide forthe proper sequence of first and second phases and with the phasedurations appropriate for the patient impedance of the patient 14.

[0048] In an alternative embodiment, the patient dependent parameterwould have already been measured prior to the step 200 using, forexample, with the ECG front end 18 equipped with circuitry capable ofmeasuring patient impedance either directly or indirectly. A low leveltest signal or a non-therapeutic pulse could be generated and thenmeasured to obtain the patient dependent parameter before delivery ofthe DBT defibrillation pulse.

[0049] In step 202 labeled MEASURE TIME TO PEAK CURRENT, the controller26 monitors the current signal during the delivery of the defibrillationpulse and determines the time interval from the initial delivery of thedefibrillation pulse to the time of the peak current. On the graph ofFIG. 5, these are times T0 and T1 respectively.

[0050] In step 204 labeled DETERMINE FIRST AND SECOND PHASE DURATIONS,the time interval from step 202 is used to determine the first andsecond phase durations. Such a determination can be made using amathematical formula or more simply by using a look up table containingthe phase duration values. The phase duration values would bepredetermined for the desired energy level such as 150 joules. As muchresolution as needed could be added to the look up table to achievedelivery of the defibrillation pulse according to the desired energylevel within specification limits.

[0051] In step 206 labeled TRUNCATE THE FIRST PHASE ACCORDING TO THEFIRST PHASE DURATION, the first phase is allowed to continue for thefirst phase duration that was determined in step 204. It is assumed thatthe first phase duration is sufficiently long to allow step 204 tocomplete in order to properly truncate the first phase according to thefirst phase duration.

[0052] In step 208 labeled DELIVER THE SECOND PHASE ACCORDING TO THESECOND PHASE DURATION, the second phase is delivered over the secondphase duration which was determined in step 204. Alternatively, a secondtime duration measurement which measures the time interval between timesT3 and T4 could be made to determine the second time interval in amanner similar to the first time interval.

[0053] Other methods of determining the patient dependent parameterduring the delivery of the DBT defibrillation pulse may be used. Forexample, the current measurement circuit 60 could contain the integratorcircuit which provides a measurement of integrated current which is afunction of total charge delivered to the patient. A time intervalmeasurement could be made of the time to reach a predetermined level ofcharge which is then used to determine the first and second phasedurations. This method is used in the Heartstream Forerunner®defibrillator to control the BTE biphasic waveform and could also beused to control the DBT defibrillation pulse of the present invention.

[0054] Another example of determining a patient dependent parameter isto measure the voltage across the energy storage capacitor 50 andtruncate the first and second phases when the voltage has dropped topredetermined percentages of the fill charge voltage. A further exampleis to measure the voltage across the pair of electrodes 16 and truncateeach phase when the charge voltage across the energy storage capacitor50 has dropped to a predetermined percentage of a maximum level.

[0055] It will be obvious to those having ordinary skill in the art thatmany changes may be made in the details of the above-described preferredembodiments of the invention without departing from the spirit of theinvention in its broader aspects. For example, a wide range ofinductance values for the series inductor, resistance values for theseries resistor, and capacitance values for the energy storage capacitormay be chosen as long as the DBT defibrillation pulse has a peak currentthat is sufficiently limited and the time interval from initiation topeak current is sufficiently long to enable an accurate measurement forthe control method explained above. Therefore, the scope of the presentinvention should be determined by the following claims.

What we claim as our invention is:
 1. A defibrillator comprising a pairof electrodes for coupling to a patient; an HV switch coupled to saidpair of electrodes; and an energy storage circuit for delivering adamped biphasic truncated defibrillation pulse through said HV switch tosaid patient.
 2. A defibrillator according to claim 1 wherein saidenergy storage circuit comprises: an energy storage capacitor coupledacross said HV switch; a series inductor and a series resistor coupledin series with said energy storage capacitor; and a shunt diode coupledacross said series inductor and series resistor.
 3. A defibrillatoraccording to claim 2 wherein said series inductor has an inductancevalue between 5 and 100 milliHenries.
 4. A defibrillator according toclaim 1 further comprising: an ECG front end coupled to said pair ofelectrodes to provide ECG information; and a controller coupled to saidfront end to receive said ECG information, to said HV switch to controla first and second phase duration of said damped biphasic truncateddefibrillation pulse, and to said energy storage circuit to receive atleast one of a current signal and a voltage signal.
 5. A defibrillatoraccording to claim 4 wherein said controller measures a patientdependent parameter according to said one of said current signal andsaid voltage signal to determine said first and second phase durations.6. A defibrillator according to claim 5 wherein said patient dependentparameter comprises a time duration between an initiation of said dampedbiphasic truncated defibrillation pulse and a peak current.
 7. Adefibrillator according to claim 5 wherein said patient dependentparameter comprises a time duration between an initiation of said dampedbiphasic truncated defibrillation pulse and a peak voltage.
 8. Adefibrillator according to claim 4 wherein said first and second phasedurations are determined according to a look up table.
 9. Adefibrillator according to claim 1 wherein said damped biphasictruncated defibrillation pulse has a peak current limited to less than amaximum value.
 10. A method for delivering a damped biphasic truncateddefibrillation pulse to a patient, comprising: coupling a defibrillatorvia pair of electrodes to said patient; initiating delivery of a firstphase of said damped biphasic truncated defibrillation pulse to saidpatient; measuring a patient dependent parameter during said delivery ofsaid first phase; and determining a first and second phase duration ofsaid damped biphasic truncated defibrillation pulse based on saidpatient dependent parameter.
 11. A method for delivering a dampedbiphasic truncated defibrillation pulse to a patient according to claim10, said measuring step comprising measuring a time interval to a peakcurrent in said damped biphasic truncated defibrillation pulse.
 12. Amethod for delivering a damped biphasic truncated defibrillation pulseto a patient according to claim 10, said measuring step comprisingmeasuring a time interval to a peak voltage in said damped biphasictruncated defibrillation pulse.
 13. A method for delivering a dampedbiphasic truncated defibrillation pulse to a patient according to claim10 further comprising: truncating said first phase according to saidfirst phase duration; and delivering a second phase of said dampedbiphasic truncated defibrillation based on said second phase duration.14. A method for delivering a damped biphasic truncated defibrillationpulse to a patient, comprising: coupling a defibrillator via pair ofelectrodes to said patient; measuring a patient dependent parameter ofsaid patient; determining first and second phase durations of saiddamped biphasic truncated defibrillation pulse based on said patientparameter; and delivering said damped biphasic truncated defibrillationpulse to said patient according to said first and second phasedurations.
 15. A method for delivering a damped biphasic truncateddefibrillation pulse to a patient according to claim 14, said measuringstep comprising measuring a patient impedance as said patient dependentparameter.
 16. A defibrillator comprising a pair of electrodes forcoupling to a patient; an HV switch coupled to said pair of electrodes;an energy storage circuit for generating a damped biphasic truncateddefibrillation pulse; and a controller coupled to said HV switch and tosaid energy storage circuit; wherein said controller initiates deliveryof said damped biphasic truncated defibrillation, measures a patientdependent parameter, and determines first and second phase durations ofsaid damped biphasic truncated defibrillation pulse based on saidpatient dependent parameter.
 17. A defibrillator according to claim 16wherein said energy storage circuit comprises: an energy storagecapacitor coupled across said HV switch; a series inductor and a seriesresistor coupled in series with said energy storage capacitor; and ashunt diode coupled across said series inductor and series resistor. 18.A defibrillator according to claim 17 wherein said series inductor hasan inductance value between 5 and 100 milliHenries.
 19. A defibrillatoraccording to claim 16 further comprising an ECG front end coupled tosaid pair of electrodes to provide ECG information to said controller todetect a shockable rhythm.
 20. A defibrillator according to claim 16wherein said controller measures said patient dependent parameteraccording to a current signal from said energy storage circuit.
 21. Adefibrillator according to claim 20 wherein said patient dependentparameter comprises a time duration between an initiation of said dampedbiphasic truncated defibrillation pulse and a peak current measured fromsaid current signal.
 22. A defibrillator according to claim 16 whereinsaid controller measures said patient dependent parameter according to avoltage signal from said energy storage circuit.
 23. A defibrillatoraccording to claim 22 wherein said patient dependent parameter comprisesa time duration between an initiation of said damped biphasic truncateddefibrillation pulse and a peak voltage measured from said voltagesignal.
 24. A defibrillator according to claim 16 wherein said first andsecond phase durations are determined according to a look up table basedon said patient dependent parameter.
 25. A defibrillator according toclaim 16 wherein said damped biphasic truncated defibrillation pulse hasa peak current limited to less than a maximum value.